1. Field of the Invention
Boundary lubrication is characterized by control of friction and wear under high load conditions; it appears to depend on the properties of the lubricant other than its viscosity. Friction under boundary lubrication conditions generally tends to be higher than that usually associated with fluid film lubrication.
The kinetic coefficient of friction, f.sub.K, is defined as the ratio of the force which resists sliding of one surface over another divided by the load between the surfaces. Under "dry"sliding conditions, coefficients of friction range from about 0.2 to 0.7. Satisfactory boundary lubrication is achieved when the coefficient of friction is reduced substantially below 0.2. The static coefficient of friction, f.sub.S, is related to the force necessary to overcome inertia and is somewhat higher than f.sub.K under both dry and lubricated conditions.
Lubricants are often called upon to perform under conditions of high ambient temperature. Frictional coefficients of lubricated surfaces should be relatively constant up to a certain ambient temperature, termed "the transition temperature", T, which for the purpose of the present invention is the temperature beyond which the kinetic coefficient of friction rises above 0.2. Clearly, a superior lubricating composition has a transition temperature substantially higher than the ambient temperature under working conditions.
Wear is much more difficult to adequately measure and predict than friction. It can vary over a large range under controlled conditions depending on the load. Gears and many machine elements require quite low wear to achieve acceptable lifetimes. The addition of extreme pressure (EP) chemical additives to a lubricating oil can increase the load-carrying capacity of the lubricant many times over. EP additives are thus of considerable economic importance to industry.
There are four mechanisms acting alone and in concert which contributes to wear; they are corrosion, fatigue, plowing, and adhesion. Adhesion occurs under conditions of nearly atomic cleanliness and is probably concomitant to plowing. Plowing wear occurs when a hard, sharp surface irregularity, or third body (e.g., a dust particle), plows through the surface, removing the boundary layers and bringing clean surfaces into contact where they may adhere. Plowing also creates more ridges and surface irregularities which undergo plastic deformation until they fatigue, fracture and leave the surface. This surface metal fatigue mechanism, with or without plowing, is evidenced by micropitting of the surface.
Corrosive wear occurs when the surface reacts chemically with its environment, e.g., metal oxides are formed by reaction with oxygen or water in the air or lubricant, or reaction with the lubricant itself. Corrosive wear is often found where chemically active additives are used to achieve EP properties, but are found to react with the surface to its detriment, e.g., sulfurized and phosphosulfurized lubricant additives can be corrosive towards nonferrous materials.
The formation of films on metallic surfaces is thermodynamically favored, but the thickness of surface films ranges from a few hundredths of a microinch for single molecule layers of absorbed gases to several dozen microinches for thick films from oils with EP additives. The problem from the boundary lubrication standpoint is to provide a boundary film with the proper chemical and physical characteristics to control friction and wear, and the correct chemical properties to avoid detrimental surface corrosion.
As the last statement implies, a balance of chemical properties is required of EP additives. While it is known that certain sulfur, phosphorus and chlorine compounds can lead to enhanced load-carrying ability and the action of these additives is partly attributed to the formation of a chemical product film on the surface, the additive must not corrode the surface of alloys containing nonferrous metals.
It is known that in the art that additives comprising chemically active constituents such as sulfur, chlorine and phosphorus, will impart extreme pressure properties to a mineral oil. Sulfurized materials, in particular, have often been used. Such materials include various oils of mineral, animal and vegetable origin.
The process of sulfurization consists of heating under suitable conditions the proper ratio of sulfur to oil. Unfortunately, the products so obtained have often disagreeable auxiliary properties, such as a tendency to sludge, corrosiveness towards nonferrous metals, especially copper, incompatibility with other oil additives, turbidity, acidity, instability and a strong odor.
Sulfurized materials found in the prior art include sulfurized simple esters of fatty acids, U.S. Pat. Nos. 2,179,061 and 2,179,065; sulfurized tall oil, U.S. Pat. No. 2,631,129; sulfurized simple esters of tall oil, U.S. Pat. No. 2,631,131; sulfochlorinated mixtures of olefins, or acids, or esters, in the presence of an epoxy compound, U.S. Pat. No. 3,316,237; and sulfurized partial ester metal salts of unsaturated dibasic carboxylic acids, U.S. Pat. No. 3,501,413.
A particularly useful and valuable additive of animal origin is sulfurized sperm whale oil, U.S. Pat. Nos. 2,179,060, 2,179,063, and 2,179,066. Recently, the Secretary of the Interior placed an embargo on the importation of sperm whale oil into the United States by assigning all of the remaining whale species to the endangered species list. Since the United States uses 25-30 percent of the world's whale products, this should have an appreciable effect on efforts to conserve the remaining specimens. Thousands of whales and over a million barrels of oil per year are involved. Sperm whale oil is an important ingredient in cutting oils, gear oils, transmission fluids, soaps and as a lubricant for precision instruments.
One of the objects of the present invention is to find a highly satisfactory and efficient substitute for sulfurized sperm whale oil in lubricants which is free of disagreeable side effects.
Another object of this invention is to provide greases and lubricant compositions of superior demulsibility.
Another object is to provide new addition agents for greases and lubricants which impart valuable oiliness properties, including high pressure, antiwear and antifriction properties.
A still further object of this invention is to provide addition agents for lubricants and greases which are noncorrosive towards nonferrous metals, especially copper.
Yet another object of this invention is to provide addition agents for greases and lubricants which are stable, clear, nonsludging and compatible with other lubrication additives.